1. Field of the Invention
The invention relates to a wet section of a machine to manufacture a fibrous material web, especially a paper or cardboard web, with a double-screen segment, in which two sieves are guided approximately parallel to one another and into which a fibrous material suspension is guided, a roll, around which the double-screen segment is looped; apparatus arranged on the roll for the immobilization of the fibrous material suspension in the doublescreen segment for the formation of the fibrous material web, and apparatus to further drain the fibrous material web.
2. Discussion of Background Information
Furthermore, the invention relates to a method for the manufacture of a fibrous material web, especially a paper or cardboard web with the steps to guide two screens in a double-screen segment approximately parallel to one another and around a roll, to introduce a fibrous material suspension into the double-screen segment, to immobilize the fibrous material suspension on the roll in the double-screen segment while forming the fibrous material web, and to drain the fibrous material web further.
Such a wet section and a method of this kind for the manufacture of a fibrous material web are generally known, e.g., from EP-A-0 607 549.
In the present context, a wet section is to be understood as the section located before the drying section of a paper or cardboard machine, that is normally the arrangement of head box, screen section, and press section. It is given that especially the screen section and the press section do not necessarily have to lie in sequence behind each other. So, for example, pressing elements can be contained in the actual screen section.
The wet section bears particular significance in the manufacture of paper webs. In the wet section, the immobilization of the fibrous material suspension, which has been supplied in fluid form, takes place. The fibers which initially float freely in the suspension thereby are "immobilized" relative to one another via the removal of water. Prior to or during the immobilization, irregularities which appear can be compensated only insufficiently or not at all through subsequent sections of the paper machine.
The construction of such a wet section is typically a compromise between different, competing demands, including web run speed, web width, regularity of the characteristics of the finished paper web, surface quality, as well as compactness of the machine, the lowest possible inclination toward contamination, easy access for cleaning, as little expense as possible, etc.
Double-screen segments are used in wet sections in so-called gap-formers as well as in so-called hybrid-formers. Two continuous screens are guided thereby over a segment, the double-screen segment, parallel to each other.
In gap-formers, the two screens are guided over a forming roll as a support body, with the fibrous material suspension being introduced from a head box into an opening formed by the two screens and lying near to the forming roll.
In known wet sections, the two screens are guided over another segment parallel to one another following the looping around the forming roll; typically, forming strips and/or draining media such as suction boxes are arranged in this other segment in order to assist the draining of the fibrous material suspension, i.e., the "immobilization," or the further draining of the fibrous material web in this other segment; see for example EP-B-0 489 094.
The noted forming strips preferably are pressed flexibly onto one of the two screens, each with a force which can be selected. It is thereby possible to optimize the so-called formation (transparency) in the finished paper.
The screen section known from EP-B-0 489 094 has a very big length, however, and exhibits a large number of individual parts.
Furthermore, a wet section in the prior art is disclosed in the German patent application 196 34 995.8.
In this wet section, similar to that in EP-A-0 607 549, an upper screen and a lower screen are looped around a forming roll and hereafter guided parallel to each other to a screen-suctioning roll, which serves further draining of the formed fibrous material web after the immobilization. In the vicinity of the suctioning zone of the screen-suctioning roll, the upper screen is lifted from the web, so that the fibrous material web remains on the lower screen.
In EP-A-0 607 549, mentioned at the outset, the two screens are guided parallel from the forming roll to a screen-suctioning roll which serves the further draining of the web. In the vicinity of its suctioning zone, the outer screen is lifted from the web and the inner screen so that the web remains behind the double-screen segment on the inner screen.